"For now, it is not certain whether North Korea is related" to the incident, Kim Eun-Hye, the presidential spokeswoman, said.

He said Lee Myung-Bak, the president, had ordered maximum efforts to rescue the crew men.

"Finding the truth [behind the incident] is important, but saving our sailors is more important," Lee was quoted as saying.

A government source quoted by the South's Yonhap news agency said officials were investigating various possible causes: an attack by a North Korean torpedo boat, a mine laid by North Korea or an explosion of munitions aboard the ship.

South Korean YTN television, quoting an unidentified presidential official, said the ship was well south of the border and North Korean ships were unlikely to be in the area.

A military source told the agency the Cheonan was holed in the stern near its propeller.

War 'unlikely'

The joint chief of staff spokesman said there were no abnormal military movements on the North Korean side of the disputed maritime border, which was the scene of deadly naval clashes in 1999 and 2002.

Paul Chamberlin, a former US naval attache to South Korea, told Al Jazeera: "If it becomes clear this was an attack from North Korea, a major escalation that would lead to general war is very unlikely."

In November the navies of the two sides exchanged fire in the area. The South's officials said a North Korean patrol boat had retreated in flames but it was not known if there were any casualties. No South Koreans were hurt.

The North refuses to accept the maritime border known as the Northern Limit Line, which was drawn up by UN forces after the 1950-53 Korean war. It says the line should run further to the south.

In January, the North fired 370 artillery shells into the sea near the border, raising tensions between the two sides.